Window onCreate fired twice for Execution#sendRedirect()

test.zul
<zk>
<window onCreate='System.out.println("ONCREATE")'></window>
<button label="hello" onClick='Executions.getCurrent().sendRedirect("/test.zul")'></button>
</zk>

Click on "hello" and it prints ONCREATE twice.
Only occurs on FF3.6.18 and IE7
Note:
1.In Firebug net panel I see one aborted GET request.
2.Removing the leading "/" in sendRedirect URL is a workaround for consistent behavior across all browsers.